I've heard many times the argument that many Torah Laws and sanctioned behaviors (such as slavery, killing entire enemy nation, marrying a rape victim, etc.) are to be understood (in an ethical sense) to be in line with the societal norms of the time the Torah was given where these ideas were well and truly embedded into all mankind.
I'm looking for sources where this is explained/discussed in the Rishonim or Acharonim. Does anyone know where it could be found?
In a different context, the Rambam has a famous theory regarding sacrifices. In the words of Reuven Chaim Klein
Maimonides explains that the Torah’s main objective is to eradicate the viewpoint of paganism. Thus, to truly understand the Torah’s original intent, one must be familiar with the philosophies and practices of ancient idolaters [...]
Taking this idea a step further, Maimonides seemingly assumes that ritual sacrifices are a sub-optimal form of worship, leading him to making the bold statement that the Torah instituted its system of ritual sacrifices to facilitate the rejection of idolatrous practices. > He explains that human nature is that whatever people have accustomed themselves to doing becomes so ingrained in their nature that it cannot be easily uprooted. Man cannot successfully transition from one extreme to the other without some time to acclimate.
See More Nevuchim 3:32 for the original, here are relevant quotes
It is, namely, impossible to go suddenly from one extreme to the other: it is therefore according to the nature of man impossible for him suddenly to discontinue everything to which he has been accustomed. [...] But the custom which was in those days general among all men, and the general mode of worship in which the Israelites were brought up, consisted in sacrificing animals in those temples which contained certain images, to bow down to those images, and to burn incense before them; religious and ascetic persons were in those days the persons that were devoted to the service in the temples erected to the stars, as has been explained by us. It was in accordance with the wisdom and plan of God, as displayed in the whole Creation, that He did not command us to give up and to discontinue all these manners of service; for to obey such a commandment it would have been contrary to the nature of man, who generally cleaves to that to which he is used [...]
For this reason God allowed these kinds of service to continue; He transferred to His service that which had formerly served as a worship of created beings, and of things imaginary and unreal, and commanded us to serve Him in the same manner [...]
By this Divine plan it was effected that the traces of idolatry were blotted out, and the truly great principle of our faith, the Existence and Unity of God, was firmly established; this result was thus obtained without deterring or confusing the minds of the people by the abolition of the service to which they were accustomed and which alone was familiar to them.
The last two cases don't seem accurate . You may not kill an enemy nation and must allow those who want to run away the ability to do so (Mechilta in Parshas Masey)You never kill women and children and always allow those who want to accept surrender terms to do so . Even by the Shiva Ammim (see Rambam Hilchos Melochim Chapter six)
Some people may question how the rest of what the Rambam says there that fits with their current moral beliefs but in that context it must be realized that (among other things) you weren't dealing with people who followed the contemporary rules of war either.
You won't find the last paragraph in the Rishonim or even Achronim because they were written before things like the Geneva Convention and the current rules of war. It would only be latter day sources who would have an issue causing them to say it.
As far as giving a rape victim the right to force her rapist to marry her (but not the other way around of course) I'm not sure what the issue with that is even by today's standards.
As far as slavery is concerned there are a few sources dating from the mid 1800s who, with some variation, say that the Torah allowed (but did not encourage) slavery because in a situation where slavery is widespread it would be better even for the slave himself to be owned by someone who has very strict limitations on how he may treat a slave (and even much stricter limitations on when he may acquire a slave who had once been a free man) than to be sold to anyone out there.
To give some perspective from my grandfather z'l who unlike anyone reading this WAS a slave : He never would have survived the six years of pure torture from when Germany invaded Poland until after WWII if not for two different few month periods when he was doing slave labor on some German commercial farm. The guy who owned the farm was no Tzadik and certainly did not follow the Torah's rules for treating an Eved Canaani. Let alone the Midas Chasidus for doing so heavily encouraged in Gemorah. But the guy also didn't kill, beat or starve his slave laborers. That "respite" allowed my grandfather the ability to "recuperate" and live. It was at that time that he came to realize that when slavery is rampant anyway you aren't doing anyone a favor by forbidding it for a limited group. Better let them have it with strict rules like the Torah does.
I do not know if this helps, but Rabbi Moshe Chaim Ephraim of Sudilkov, the grandson of the Baal Shem Tov writes in his sefer "Degel Machaneh Ephraim, Parshas Bereishis" that in every generation, the holy scholars (of that generation) are "making up" the Torah since the Torah is interpreted according to the needs of that generation:
In every generation, the scholars (of that generation) are making up [alternative translation: are finishing, or filling up] the Torah, because the Torah is being interpreted in every generations according to the needs of that generation And according to the source of their souls - so is God enlightening the eyes of the wise people of that generation
Similary, the Kedushat Levi writes:
This is a tremendous power that G’d has granted to us, i.e. that the righteous people who will in different generations reveal these “new” aspects of the Torah will become an integral part of the Torah.
